(1) Field of the Invention
The present invention relates generally to pressure barriers for circular waveguides, and more particularly to a broadband pressure barrier for circular waveguides capable of electromagnetically passing a broad passband of radio frequency energy while also serving as a mechanical sealing barrier.
(2) Description of the Prior Art
In underwater waveguides, pressure barriers must mechanically block the passage of water when flooded while allowing electromagnetic energy to pass therethrough with minimal reflection when in the dry state. The dimensions of the barrier (i.e., its thickness) and the type of material used determines the mechanical, or hydrostatic pressure blocking ability. There are a number of materials that can be used for this purpose. In most cases, however, electrical characteristics and mechanical characteristics are not compatible. For example, if a single disk barrier were placed in a circular waveguide, it would have a nominal thickness of a half-wavelength at the center of the passband of interest. If the disk has a large relative permittivity (greater than 5), the disk may be capable of passing electromagnetic energy, but the thickness of the disk may not be sufficient to withstand the expected pressure. On the other hand, a disk fabricated from a material with a low relative permittivity (less than 3) will be thicker and better able to withstand the expected pressures, but the electrical characteristics may be unsuitable for the passage of electromagnetic energy.
In the past, pressure barriers for circular waveguide applications comprised a series of cascaded disks as shown in the barrier 10 is sealed in a cylindrical waveguide 100 and consists of three cascaded disks 11, 12 and 13. Low permittivity disks 11 and 13 allow for impedance matching between the empty waveguide and high permittivity disk 12 such that barrier 10 minimizes the reflection from an impinging electromagnetic field. Disks 11 and 13 are nominally a quarter-wave thick (measured with respect to wavelength in waveguide 100) at the center of the passband of operation. This arrangement can meet the requirement of low reflection and resistance to collapse under high hydrostatic pressure if the center or high-permittivity disk 12 is mechanically capable of doing so. The difficulty in this arrangement is that low permittivity disks 11 and 13 must be manually tuned so that reflection is minimized. This is performed by manually grinding or lapping the disks, and assembling the barrier to check the voltage standing wave ratio (VSWR), from which the reflection coefficient is computed. If the VSWR is not within specified limits, the barrier is disassembled, and the tuning process is repeated--a time consuming and expensive procedure.
Another pressure barrier design offering low reflection to an incident propagating field is shown in the axial cross-section of FIG. 2 where two pressure barrier disks 20 and 21 are inserted in circular waveguide 100. However, to be effective, tolerances between disks 20 and 21 must be kept on the order of 0.0002 to 0.0005 inches. This makes the barrier manufacturing process difficult and expensive. If a small VSWR is required, more disks are cascaded in a similar spaced apart fashion. The disks can differ in permittivity and can be spaced to yield a minimum VSWR across the band of interest. Although effective in reducing the VSWR, spacing tolerances increase the level of complexity. Another problem associated with the multiple disk system is that the relative permittivities of the disks can vary thereby adding a design parameter to be considered.
U.S. Pat. No. 3,439,296 is an example of a multiple disk system. A dielectric structure formed by a pair of axially spaced dielectric plates is sealed across a waveguide. An inductive iris structure having a diameter smaller than the waveguide diameter is disposed between the plates. Such iris structures suppress extraneous electromagnetic waveguide modes (i.e. "ghost modes") that are evanescent in the vicinity of a microwave window structure. (The ghost mode fields have a detrimental effect on the high-power handling ability of the disk.) The iris structure alters the radius of the waveguide, making a waveguide carrying the non-dominant TE.sub.01 mode subject to mode conversion.
It is further known in the prior art to thin or thicken a dielectric disk along longitudinal regions thereof where the electric field is at its maximum intensity. For example, in U.S. Pat. No. 3,594,667, the dielectric disk is altered in a fashion that improves the high-power operating bandwidth of the disk. The thickness of the dielectric material or the dielectric constant of the window member are varied as a function of the distance from the center of the window. The objectives of the alterations are met by tuning out "ghost modes" in the dielectric region. The extent of the alterations is arrived from a knowledge of the electric field configuration possessed by the "ghost modes".